OG reference field widget update: MR #570 did not properly merge, leaving og_complex in place

I did my best to merge the diff from https://github.com/Gizra/og/pull/570, there are a bunch of todo's we should fix before merging this, and we should add a way to transition anybody who is currently using og_complex

Re-titling to match https://github.com/Gizra/og/issues/160

I was testing this out and found a bug when validating the Groups audience field.
Steps to reproduce:
1. Create a group
2. Pick another content type, and associate it with the group in the first step. I picked the Article node. And go to Manage form display for Article and change the widget from Select list to Autocomplete (Select list seems buggy and will not show anything)
3. Create a new test user and add it to the group.
4. As an admin, go to admin>configuration>organic groups>og roles> and edit the roles for the group you created. Under Group content check Article: Edit any content (since I selected Article in step 2) Side note: there might also be a bug here because if you just gave the test user permission to edit any Article, it seems that they are not able to edit the group content, which is an Article, without doing this step.
5. Also as an admin, give the test user the permission Article: Edit any content
6. Now log in as the test user and create a new Article and associate it with the group.
7. As the test user, edit the content and then try to save the node. You will see an error that looks like this:
This entity (node: 1) cannot be referenced.
and it is a validation error for the Groups audience field.
Expected result:
The test user should have been able to save that group content and not see that error.

I added a test for #5

Let's target 2.x and NOT backport, so we don't break any site which uses 8.x-1.x. Please change the base branch of MR, I have no permissions

joelpittet → changed the visibility of the branch 8.x-1.x to hidden.